Drug delivery devices are generally known to be used for the administration of medicinal products, particularly fluid medicaments, for example insulin or heparin, but also for other medicinal products for self-administration by a patient. Often, drug delivery devices are pen-type injectors which dispense a pre-set dose of a fluid medicinal product.
Prior to the first use of a drug delivery device, the drug delivery device usually has to be primed. During a priming-step gaps which are contained in the drug delivery device between components may be closed, particularly gaps between components which are involved in the mechanism for dispense of the drug contained in the drug delivery device (for example a gap between a piston rod and a cartridge bung). These gaps may be a consequence of the tolerances associated with all the assembled parts which may occur through the manufacturing of the device and the requirement not to preload the bung axially in the assembled device. Usually, all gaps between components being involved in the mechanism of the drug delivery device are closed, particularly the gap between the piston rod and the cartridge bung. However, in certain embodiments reducing the gaps to a certain extent may also be possible. After having performed the priming-step, the device is in a primed condition so that the user can now dispense the intended dose exactly, including the first dispensed dose.
A problem of such drug delivery devices is that the users of the drug delivery device who are not familiar with such devices may fail to or incorrectly prime the device before dispensing the first dose. If this occurs, the user may inject the prime fluid or the correct volume of medicament may not be delivered in the first dose. In particular, often the user does not know whether he has primed the device before an intended injection of a dose. Usually, the unfamiliar user cannot see if he has to perform a priming-step before using the device in the correct way.